The Mystery of the Missing Brown Dwarfs

Take a crystal glass and drop it on the floor, you’ll end up with a few large pieces, a greater number of smaller shards, and lots and lots of tiny bits. For decades, astronomers assumed the same pattern would hold for interstellar clouds breaking up into stars. Large collapsing gas clouds — stellar birthplaces — were well known to break up into a few massive stars, more medium-sized ones like the Sun, and lots of low-mass red dwarfs. That's the pattern we see not just among stars born together in a cluster, but also among the stars scattered throughout our neighborhood of space generally.

Would this trend extend to even smaller stellar fragments, known as brown dwarfs? These are objects with masses too low for them to ignite hydrogen fusion at their centers and light up as actual stars. (They're often defined as any object having between 13 and 75 Jupiter masses.) At some lower limit, the cloud-breakup process should become inefficient and cease to form smaller fragments. Or at least not so many.

But where was that lower limit? Brown dwarfs are extremely dim and hard to find, much less study. Astronomers confirmed the first ones only in 1995, as modern infrared technology came on line. So no one knew where the bottom would be. Was interstellar space full of these tiny star-formation bits? Would they outnumber all visible stars put together?

No, it turns out. At this week's American Astronomical Society meeting in Long Beach, California, astronomers described the most comprehensive survey yet of objects in the "solar neighborhood," the region within 10 parsecs (32.6 light-years) of the Sun. The survey is part of an ongoing project known as RECONS, the Research Consortium on Nearby Stars. It has added a lot of dim bulbs to the tally of our near neighbors. The list within 10 parsecs now includes 239 red dwarf stars — but only 12 brown dwarfs.

More could be hiding that are still too cool and faint, but there can't be many. Astronomers seem to have found the turnaround where ever-tinier fragments become fewer in number with declining mass rather than more numerous (see graph above).

RECONS scientists also announced a different brown-dwarf finding at the meeting. Not only are there relatively few of these objects floating in isolation, there are almost none orbiting normal stars. The reason for this so-called "brown-dwarf desert" among binary companions is a mystery.

It's not new. Astronomers doing radial-velocity hunts for objects closely orbiting stars (by looking for their gravitational tugs) have turned up lots of giant planets but almost no companions in the brown-dwarf mass range.

Those hunts, however, have mostly been sensitive to objects that orbit within 5 astronomical units of the host star (Jupiter’s distance from the Sun). To look for brown dwarfs orbiting at much wider distances, RECONS astronomers took deep, high-resolution infrared images of 233 nearby stars using the Hubble Space Telescope. Although double stars are extremely abundant, the team found only one double in which the companion is a brown dwarf. (And that object, interestingly, turned out to be double itself.)

“Brown dwarfs are running out of places to hide; they’re just not there,” sums up RECONS team member Sergio Dieterich (Georgia State University), who led the search for brown-dwarf companions. “The deeper we look, the fewer of them we find.”

For reasons unknown, nature has an especially hard time forming a brown dwarf in tandem with a normal star. “We have seeds in interstellar gas clouds, and they continue to sprout to form stars, but they don’t form a lot of brown dwarfs,” says RECONS team leader Todd Henry (Georgia State University).

However, this odd binary taboo does not extend to pairs of two brown dwarfs. Such low-mass binaries are actually turning out to be fairly common among the brown-dwarf population generally. An example is the pair Kelu-1, resolved in the infrared Hubble image above.

See the RECON group's press release for more.